Shorter wavelength photo-annealing apparatus for rare-earth-doped fiber and its optical assemblies under irradiation

ABSTRACT

An optical fiber apparatus is suitable to operate under irradiation, more particularly to mitigating the damage of a rare-earth-doped optical fiber element as part of an optical fiber assembly causes by irradiation. The irradiation mitigation attributes to a photo-annealing apparatus including at least a shorter wavelength photo-annealing spectral content, which is relative to that of a pump light source, for effectively photo-annealing the rare-earth-doped fiber element. Photo-annealing by such shorter wavelength light results in a fast and nearly complete recovery of radiation induced attenuation of the rare-earth-doped optical fiber element in the wavelength range from 900 nm to 1700 nm.

FIELD OF THE INVENTION

The invention relates to an optical fiber assembly suitable to operateunder irradiation environment. More particularly this invention is tomitigate the radiation damage of a rare-earth-doped optical fiberelement as part of the optical fiber assembly. The mitigation ofradiation damage attributes to a photo-annealing apparatus in which thewavelength range of the photo-annealing light source is less than orequal to the wavelength of the pump light source for therare-earth-doped fiber.

BACKGROUND OF THE INVENTION

Space technology plays an increasingly important role in our daily life.However, spacecraft is typically hard or expensive to reach for serviceafter launch. Therefore, components, sub-assemblies and systems forspace applications require special design and rigorous test in order toperform over space environment, especially irradiation.

Due to the advantages of light weight, compact size, broad bandwidth,and resistance of electromagnetic interference, fiber optics systems andassemblies have been developed and employed for space and nuclearfacilities applications including communication, sensing, navigationsand etc. However, it has been known that the performance of anoptical-fiber-based device would be severely degraded by the RIA(Radiation Induced Attenuation). The RIA could cause severe output powerloss of the optical-fiber-based device, and hence greatly limits itsoperating life. To decrease RIA, an effective method needs to bedeveloped.

In the prior art, some methods have been reported to reduce the RIA inpassive single-mode fibers (no rare-earth dopant) through thermalannealing (U.S. Pat. No. 4,229,069), and hydrogen pre-loading approaches(U.S. Pat. No. 6,130,981). However, the thermal annealing approach needsto raise temperature up to 200° C. to 300° C. to decrease RIAeffectively. Such a high temperature environment could damage or degradethe device and its associate assembly. On the other hand, the hydrogenpre-loading method needs a hermetic coating to avoid out-diffusion ofhydrogen, and the fabrication process of a hermetic coating might becomplicated and expensive.

In addition, prior to the present invention, photo-annealing method hadbeen reported, with a limited success, to reduce the RIA in both passivesingle-mode fibers such as a pure-core fiber and a Ge-doped fiber (U.S.Pat. No. 4,232,228), and an active Er-doped fiber. Moreover, the priorphoto-annealing approach for active Er-doped fiber employed only 980-nmlight source.

According to the present invention, applicants have departed from theconventional wisdom, and had conceived and implemented a photo-annealingapparatus to include at least a shorter wavelength photo-annealingspectral content, which is relative to that of a pump light source, foreffectively photo-annealing the rare-earth-doped fiber element. Thephoto-annealing by shorter wavelength light results in a fast and nearlycomplete recovery of RIA of the rare-earth-doped optical fiber elementin the wavelength range from 900 nm to 1700 nm. Such a fast and nearlycomplete RIA recovery is unprecedented in open literature. The inventionis briefly described as follows.

SUMMARY OF THE INVENTION

To resolve the problems of RIA associated with an optical fiber assemblyconsists of at least a rare-earth-doped fiber element effectively, thepresent invention provides a new and useful photo-annealing apparatusconfigured to achieve a fast and nearly complete recovery of RIA.

The photo-annealing apparatus of the present invention consists of anoptical fiber assembly and a photo-annealing light source. The opticalfiber assembly, including a rare-earth-doped optical fiber device whichis connected to the photo-annealing light source. Preferably, thewavelength range of the photo-annealing light is less than or equal tothe wavelength of exciting rare-earth elements such as Erbium orYtterbium. When the light from the photo-annealing source is coupled tothe optical fiber assembly, it can recover the RIA of therare-earth-doped optical fiber more than 50%, preferably at least 75%,and most preferably at least 99% in the wavelength range from 900 nm to1700 nm. Relative to the conventional photo-annealing method, thepresent invention of employing a relatively shorter photo-annealingwavelength, as compared to the wavelength for exciting therare-earth-doped fiber element, provides a nearly complete RIA recoveryfrom 900 nm to 1700 nm range, and at least a two order of magnitudeimprovement in RIA recovering time. Such a fast and nearly complete RIArecovery makes the invented apparatus extremely suitable for space andnuclear facilities applications including communication, sensing,navigations and etc.

In accordance with one aspect of the present invention, an optical fiberassembly is provided. The optical fiber assembly is connected to aphoto-annealing light source, and including at least a rare-earth-dopedfiber element and having a first port and a second port, wherein thefirst port and the second port can be an input, output or an unusedport. The photo-annealing light source is coupled to the optical fiberassembly to recover the RIA.

Preferably, the photo-annealing light source is a laser or a broadbandlight source.

Preferably, the wavelength range of the photo-annealing light source isless than or equal to the wavelength for exciting rare-earth-doped fiberelement in the optical fiber assembly.

Preferably, the recoverable RIA covers all of the spectral range from900 nm to 1700 nm. And the photo-annealing apparatus recovers more than50%, preferably at least 75%, and most preferably at least 99% of RIAarising from rare-earth-doped fiber element.

Preferably, the photo-annealing light source operates continuously orintermittently.

In accordance with further aspect of the present invention, an opticalfiber assembly is provided. The optical fiber assembly includes a lightsource coupled to an optical fiber assembly including at least arare-earth-doped fiber element. The light source photo-anneals theradiation induced defects of the rare-earth-doped fibers and pumps therare-earth-doped fiber device simultaneously.

Preferably, the light source is a laser or a broadband light source.

Preferably, the light source is a photo-annealing light source andfurther is a pump light source for exciting rare-earth elements.

Preferably, the light source recovers the RIA of the rare-earth-dopedfiber and pumps the rare-earth-doped fiber simultaneously.

Preferably, the wavelength range of the light source is less than thelongest wavelength which is able to excite the rare-earth ions in theoptical fiber assembly.

Preferably, the recoverable RIA covers all of the spectral range from900 nm to 1700 nm. And the photo-annealing apparatus recovers more than50%, preferably at least 75%, and most preferably at least 99% of RIAarising from rare-earth-doped fiber element.

In accordance with one aspect of the present invention, aphoto-annealing method is provided. The photo-annealing method includesthe steps of providing a photo-annealing light source and an opticalfiber assembly including at least a rare-earth-doped fiber; transmittingthe light from the photo-annealing light source to the optical fiberassembly; and recovering RIA of the rare-earth-doped fiber by thephoto-annealing light source.

The above aspects and advantages of the present invention will becomemore readily apparent to those ordinarily skilled in the art afterreviewing the following detailed descriptions and accompanying drawings,in which:

BRIEF DESCRIPTION OF THE DRAWINGS

A schematic diagram of a general optical fiber apparatus is shown inFIG. 1;

FIGS. 2A and 2B is a first and a second preferred implementation caseschematic diagram of the optical fiber apparatus, respectively, whereinFIG. 2A is a single-pass backward configuration and FIG. 2B is adouble-pass backward configuration;

FIGS. 3A and 3B is a third and a forth preferred implementation caseschematic diagram of the optical fiber apparatus, respectively, whereinFIG. 3A is a single-pass backward configuration and FIG. 3B is adouble-pass backward configuration;

FIG. 4 is a fifth preferred implementation case schematic diagram of theoptical fiber apparatus; and

FIG. 5 is the absorption spectra of an Erbium-doped fiber employed inFIG. 4 configuration, and were measured when (a) irradiated up to 129.2krad under ⁶⁰Co irradiation, (b) self-recovered after 5.5 months at roomtemperature, (c) photo-annealed by 976-nm laser with 290 mW for 98minutes, (d) photo-annealed by 532-nm laser with 10 mW for 103 minutes,and (e) was before ⁶⁰Co irradiation.

FIG. 6 shows that RIA of EDF at wavelength of 950 nm varied with timewhen photo-annealed by (a) a 532-nm laser with 10 mW and (b) a 976-nmlaser with 290 mW.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically withreference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for the purposes of illustration and description onlyit is not intended to be exhaustive or to be limited to the precise formdisclosed.

Please refer to FIG. 1, which is a general schematic diagram of theoptical fiber apparatus showing an optical fiber assembly connected to aphoto-annealing light source. The optical fiber apparatus, as describedin FIG. 1, includes a photo-annealing light source 100 coupled to anoptical fiber assembly 102. The optical fiber assembly 102 includes atleast a rare-earth-doped fiber. Preferably, the optical fiber assembly102 includes WDMs (Wavelength Division Multiplexings), pump lasers forexciting rare-earth-doped fibers, optical isolators, plural opticalfibers and having a first port 1031 and a second port 1032, wherein thefirst port 1031 and the second port 1032 can be an output port, an inputport or an unused port. It has been known that the performance of arare-earth-doped fiber device would be severely degraded by RIA. The RIAincludes optical absorption bands of the corresponding radiation induceddefect which can be diminished by a photo-annealing light. Thephoto-annealing light source 100, which can be a laser or a broadbandlight source, generates a light incident into the rare-earth-doped fiberof the optical fiber assembly 102 to reduce the RIA. Preferably, thewavelength of the photo-annealing light is ranged from 355 nm to 980 nm,and the RIA at least from 900 nm to 1700 nm in wavelength can beefficiently recovered. Consequently, the performance of the opticalfiber assembly 102 could be recovered by the photo-annealing lightsource 100.

Please refer to FIG. 2A, which is a first preferred implementation caseschematic diagram of the present invention, showing a photo-annealinglight source 200 coupled to an optical fiber assembly 201. The opticalfiber assembly 201 is used as a single-pass backward configuration,including a 3-port WDM 202 connected to an optical isolator 204 with anoutput port 205, a rare-earth-doped fiber 203 connected to the 3-portWDM 202 and a fiber termination 206, and a fiber splicing 207.Preferably, a rare-earth-doped fiber 203 is an EDF (Erbium Doped Fiber).The photo-annealing light source 200 is also an optical pump lightsource for exciting the rare-earth-doped fiber 203. When a light fromthe photo-annealing light source 200 is coupled to the optical fiberassembly 201, it can photo-anneal the radiation-induced defects of therare-earth-doped fiber 203 and pump the rare-earth-doped fiber 203simultaneously. In such an architecture, the transmitting direction ofthe output ASE (Amplified Spontaneous Emission) light is opposite tothat of the pump light. The RIA of the rare-earth-doped fiber 203 couldnearly be diminished in the wavelength range from 900 nm to 1700 nm bythe photo-annealing light source 200.

Please refer to FIG. 2B, which is a second preferred implementation caseschematic diagram of the present invention, showing a photo-annealinglight source 200 coupled to an optical fiber assembly 201. The opticalfiber assembly 201 is used a double-pass backward configuration,including a 3-port WDM 202 connected to an optical isolator 204 with anoutput port 205, a rare-earth-doped fiber 203 connected to the 3-portWDM 202 and a reflector 206, and a fiber splicing 207. Preferably, arare-earth-doped fiber 203 is an EDF. The photo-annealing light source200 further acts as an optical pump light source for exciting therare-earth-doped fiber 203. When a light from the photo-annealing lightsource 200 is coupled to the optical fiber assembly 201, theradiation-induced defects of the rare-earth-doped fiber 203 can bephoto-annealed and the rare-earth-doped fiber 203 also can be pumped. Insuch an architecture, the forward ASE light of the rare-earth-dopedfiber 203 excited by the photo-annealing light source 200 is reflectedby the reflector 206, and re-amplified by the pumped rare-earth-dopedfiber 203. The transmitting direction of the output ASE light isopposite to that of the pump light. The RIA of the rare-earth-dopedfiber 203 could nearly be diminished in the wavelength range from 900 nmto 1700 nm by the photo-annealing light source 200.

Please refer to FIG. 3A, which is a third preferred implementation caseschematic diagram of the present invention, showing a photo-annealinglight source 300 coupled to an optical fiber assembly 301. The opticalfiber assembly 301 is used as a single-pass backward configuration,including a first 3-port WDM 3041 connected to a pump laser 302, asecond 3-port WDM 3042 connected to an optical isolator 306 with anoutput port 307, a rare-earth-doped fiber 305 connected to the 3-portWDM 3042 and a fiber termination 303, and a fiber splicing 308.Preferably, a rare-earth-doped fiber 305 is an EDF. When a light fromthe photo-annealing light source 300 emits to the optical fiber assembly301, it can recover the RIA of the rare-earth-doped fiber 305. The pumplaser 302 is used to excite the rare-earth-doped fiber 305 forgenerating ASE light. Therein, the light from the photo-annealing lightsource 300 can operate continually or intermittently. In such anarchitecture, the transmitting direction of the output ASE light isopposite to that of the pump light. The RIA of the rare-earth-dopedfiber 305 could nearly be diminished in the wavelength range from 900 nmto 1700 nm by the photo-annealing light source 300.

Please refer to FIG. 3B, which is a forth preferred implementation caseschematic diagram of the present invention, showing a photo-annealinglight source 300 coupled to an optical fiber assembly 301. The opticalfiber assembly 301 is used as a double-pass backward configuration,including a first 3-port WDM 3041 connected to a pump laser 302, asecond 3-port WDM 3042 connected to an optical isolator 306 with anoutput port 307, a rare-earth-doped fiber 305 connected to the 3-portWDM 3042 and a reflector 303, and a fiber splicing 308. Preferably, arare-earth-doped fiber 305 is an EDF. When a light emitted from thephoto-annealing light source 300 is coupled to the optical fiberassembly 301, it can recover the RIA of the rare-earth-doped fiber 305.The pump laser 302 is used to excite the rare-earth-doped fiber 305 forgenerating ASE light. Therein, the light from the photo-annealing lightsource 300 can operate continually or intermittently. In such anarchitecture, the forward ASE light of the doped fiber 305 excited bythe photo-annealing light source 300 is reflected by the reflector 303,and re-amplified by the pumped rare-earth-doped fiber 305. Thetransmitting direction of the output ASE light is opposite to that ofthe pump light. The RIA of the rare-earth-doped fiber 305 could nearlybe diminished in the wavelength range from 900 nm to 1700 nm by thephoto-annealing light source 300.

Please refer to FIG. 4, which is a fifth preferred implementation caseschematic diagram of the present invention, showing a photo-annealinglight source 400 coupled to an optical fiber assembly 401, including a3-port WDM 404 connected to a pump laser 402, an optical isolator 403with an output port 406, a rare-earth-doped fiber 405 connected to thephoto-annealing light source 400, and a fiber splicing 407. Preferably,a rare-earth-doped fiber 405 is an EDF. When a light emitted from thephoto-annealing light source 400 is coupled to the optical fiberassembly 401, it can recover the RIA of the rare-earth-doped fiber 405.The pump laser 402 is used to excite the rare-earth-doped fiber 405 forgenerating ASE light. Therein, the light from the photo-annealing lightsource 400 can operate continually or intermittently. In such anarchitecture, the transmitting direction of the output ASE light isopposite to that of the pump light. The RIA of the rare-earth-dopedfiber 405 could nearly be diminished in the wavelength range from 900 nmto 1700 nm by the photo-annealing light source 400.

Please refer to FIG. 5, which was measured under the configuration ofFIG. 4. The absorption spectra of EDF show that the RIA could nearly bediminished in the wavelength range from 900 nm to 1700 nm by thephoto-annealing of 532-nm laser. Wherein the absorption spectrum of (d)is measured under a 532-nm laser with 10 mW for 103 minutes, and theabsorption spectrum of (e) is measured before ⁶⁰Co irradiation. Asshown, these two measured curves are nearly coincident with each othermeans that the RIA of EDF is nearly recovered.

Please refer to FIG. 6, the RIA recovering times by photo-annealing ofthe 532-nm and 976-nm lasers are compared. The photo-annealing effect ofthe 532-nm laser was pronounced. The needed times of half recovered RIAwas found 5.6 seconds and 2679 seconds for the 532-nm using lower inputpower of 10 mW and 976-nm lasers using higher input power of 290 mW,respectively. This is two order of magnitude of difference in RIArecovering time. Therefore, the short wavelength laser of 532-nm showedexcellent annealing rates on the EDF when compared with the longerwavelength laser of 976-nm.

In conclusion, by means of the photo-annealing light source, the opticalfiber assembly can mitigate the degradation by the RIA of therare-earth-doped fibers under the irradiation environment. The lightemitted from the photo-annealing light source, especially for thewavelength of the light is less than the pump wavelength of excitingrare-earth elements, is able to recover the RIA in fast annealing ratesso that the optical fiber assembly can maintain its performance as ifoperating in a non-radiation environment. Therefore, the photo-annealingapparatus of the present invention is particularly suitable foroperating at the irradiation environments, such as space, nuclear powerplant facilities and etc.

Based on the above descriptions, it is understood that the presentinvention is indeed an industrially applicable, novel and obvious onewith values in industrial development. While the invention has beendescribed in terms of what are presently considered to be the mostpractical and preferred embodiments, it is to be understood that theinvention should not be limited to the disclosed embodiment. On thecontrary, it is intended to cover numerous modifications and variationsincluded within the spirit and scope of the appended claims which are tobe accorded with the broadest interpretation so as to encompass all suchmodifications and variations. Therefore, the above description andillustration should not be taken as limiting the scope of the presentinvention which is defined by the appended claims.

1. An optical fiber apparatus, comprising: an optical fiber assemblyincluding at least a rare-earth-doped fiber element and having a firstport and a second port, wherein the first port and the second port canbe an input port, an output port or an unused port; a photo-annealinglight source coupled to the optical fiber assembly and emitting a lightto recover an optical loss induced by irradiation; and a pump lasercoupled to the optical fiber assembly and emitting a light to pump therare-earth-doped fiber element.
 2. The optical fiber apparatus asclaimed in claim 1, wherein the photo-annealing light source is either alaser or a broadband light source.
 3. The optical fiber apparatus asclaimed in claim 1, wherein the wavelength range of the photo-annealinglight source is less than the wavelength of the pump laser which is usedto excite rare-earth-doped fiber elements.
 4. The optical fiberapparatus as claimed in claim 1, wherein the photo-annealing lightsource is able to recover more than 50%, preferably at least 75%, andmost preferably at least 99% of RIA in any wavelength interval from 900nm to 1700 nm.
 5. The optical fiber apparatus as claimed in claim 1,wherein the photo-annealing light source emits light continually orintermittently.
 6. An optical fiber apparatus, comprising: an opticalfiber assembly including at least a rare-earth-doped fiber element; anda light source, coupled to the optical fiber assembly, emits a light torecover a radiation induced attenuation and pumps the optical fiberassembly simultaneously.
 7. The optical fiber apparatus as claimed inclaim 6, wherein the light source is a photo-annealing light source. 8.The optical fiber apparatus as claimed in claim 6, wherein the lightsource further is an optical amplifying light source for excitingrare-earth-doped fiber elements.
 9. The optical fiber apparatus asclaimed in claim 6, wherein the light source is either a laser or abroadband light source.
 10. The optical fiber apparatus as claimed inclaim 6, wherein the wavelength range of the light source is less thanthe wavelength of excited rare-earth-doped fiber elements.
 11. Theoptical fiber apparatus as claimed in claim 6, wherein the light sourcewhich serves as a photo-annealing and pump light source at the same timerecovers more than 50%, preferably at least 75%, and most preferably atleast 99% of RIA in any wavelength interval from 900 nm to 1700 nm. 12.A photo-annealing method, comprising: generating a continuous orintermittent light to recover more than 50%, preferably at least 75%,and most preferably at least 99% of RIA of a rare-earth-doped fiber inany wavelength interval from 900 nm to 1700 nm.
 13. The photo-annealingmethod as claimed in claim 12, further includes the following steps:providing a photo-annealing light source, and an optical fiber assemblyincluding at least a rare-earth-doped fiber; transmitting the light fromthe photo-annealing light source to the optical fiber assembly; andrecovering RIA of the rare-earth-doped fiber by the photo-annealinglight source.